An activating mechanism may be used to trigger a process when a preselected event occurs, for example, an increase in pressure. The type of device to be triggered may be an in-line plug in a pipeline. It is particularly desirable that the activating mechanism be reliable and inexpensively constructed. It will be apparent from the following description that the present invention completely fulfills these requirements. Related applications Ser. No. 113,985 filed Jan. 21, 1980; Ser. No. 135,835 filed Mar. 31, 1980, now abandoned; No. 218,157 filed Dec. 19, 1980, now abandoned; and No. 218,233 filed Dec. 19, 1980, now abandoned are incorporated herewith.